Load responsive switch apparatus



Jan; 31, 1967 J, PlECHOTTA I 3,302,142

LOAD RESPONSIVE SWITCH APPARATUS Filed March 26, 1964 2 Sheets-Sheet 143 11c. 29 POWER Q SOURCE 40 i I I I 9* i i 0 t l 331' 25 v I I I 26 I lI l 30 D 42 If n T 4/ 44 7 I I l 46 INVENTOR.

1 J09? MAX PIECHOTTA 2/ 0 BY ndrus Star 2 20 7 i K ATfomvEYs Jam 1967 J.M- PIECHOTTA LOAD RESPONSIVE SWITCH APPARATUS Filed March 26, 1964 2Sheets-Sheet 2 I'NVENTOR.

JOSEF MAX PIECHOTTA BY find/"us Star/g2 United States Patent 3,302,142LOAD RESlPONSIVE SWITCH APPARATUS Josef Max Piechotta, 2138 S. 18th St.,Milwaukee, Wis. 53205 Filed Mar. 26, 1964, Ser. No. 357,009 2 Claims.(Cl. 335128) This invention relates to an electrical load responsiveswitch apparatus and particularly to such a switch apparatus foroperating an indicator or recording means in accordance with theoperating timeof an arc welding unit or the like. This application is acontinuation-in-part of applicants copending application entitled LoadResponsive Switch Apparatus filed December 10, 1962 with Serial No.243,565.

In arc welding and arc cutting shops, a record of the actual weldingtime during a given base period may be maintained for various reasons.The record may be employed for supervisory purposes, efficiency studies,calculation of wages or for any other purpose desired. Many differentindicating systems have been heretofore suggested. For example, USPatent 2,340,114 discloses an indicator for use in a welding circuit forrecording the operating time therein. In accordance with that patent, agenerally conventional relay is employed to open and close the circuit.

Indicators for shop practice must of necessity be rugged, reliable unitswhich can withstand the atmospheric conditions as well as theconventional shop handling and use inherently encountered in the shop.Further, in large shops Where a relatively great number of smallindividual machines are in use, the recording and pickup structureshould be of a relatively simple and eflicient design to minimize costof installation, maintenance and servicing.

The present invention is particularly directed to a very rugged andreliable switch structure of a relatively simple and inexpensiveconstruction and installation.

In accordance with the present invention, a magnetic member generallydefining a channel-shaped passageway is provided with a movable portionoverlying or spanning the opening to the channel-shaped member. Themovable portion is resiliently coupled to directly actuate a suitableswitch. A portion of the main load conductor, preferably in the form ofa separate metal strip, is passed through the channel-shaped portion.When the welding process is initiated, the load current flowing throughthe conductor establishes a magnetic field which is concentrated withinthe U-shaped magnetic frame and the movable portion spanning the openingthereof. At a preselected current level, the movable portion isattracted to complete the magnetic path in accordance with knownphenomenon and thereby moves the switch to a closed or recordingposition. The resilient mounting of the switch unit allows variations ofthe pull-in current and thus allows adjustment of the switch unit torecord only welding operations of a predetermined load characteristic.

The present invention may be constructed to respond to either analternating or a direct current load current. In either event, thestructure preferably includes a core having a U-shaped laminated coreportion formed by a plurality of correspondingly figured laminationplates riveted or otherwise suitably interconnected. A relatively thin,plate-like bridge member is pivotally secured to the one arm of theU-shaped core and bridges the gap between the arms with the free end ofthe bridge member aligned with and overlying the end of the opposite armof the U-shaped core portion. The laminated structure is employed topermit adjustment of the switching apparatus to preselected currentranges as well as to provide better response and action to the currentlevels. Within each current range, the precise triggering or actuatingcurrent isdetermined by a spring tension setting of the bridge 3,302,142Patented Jan. 31, 1967 member as well as adjustment of the air gapbetween the bridge member and the core.

In providing a direct current responsive unit, the bridge member isprovided with small non-magnetic spacing means secured to the inner faceof the free end to positively prevent complete interengagement with theopposing face of the U-shaped core and closing of the air gap. Thisprovides a very rapid response and breaking of the circuit by movementof the bridge plate in response to a decrease in holding current.

In an alternating current circuit, it has been found that a shorted turnshould be wound about a portion of the U-shaped core immediatelyadjacent the free end of the bridge plate and the adjacent end of theU-shaped core. This provides very silent and reliable operation.

Applicant has found that in all cases, the AC. and DO relays can beformed with a basic similar construction with the current rangescontrolled by the number of laminations employed in the U-shaped core.In A.C. circuits, looping of the load current conductor to form awinding of one, two or more turns also provides response to differentcurrent levels. In relatively high current ranges, however, the loadconductor may pass directly through the U-shaped core structure andprovide highly reliable operations.

The present invention thus provides a rugged and reliable currentresponsive switch which is of an economical construction and requires aminimum of installation, maintenance and servicing cost.

The drawings furnished herewith illustrate the best mode presentlycontemplated for carrying out the invention.

In the drawings:

FIG. 1 is a schematic circuit diagram incorporating a switch constructedin accordance with the present invention;

FIG. 2 is an enlarged front elevational view of a switch assembly shownin FIG.1;

FIG. 3 is a side elevational view of FIG. 2;

FIG. 4 is a view similar to FIG. 3 showing an alternative constructionof a direct current responsive relay for incorporation in any suitabledirect current power circuit;

FIG. 5 is a fragmentary view of FIG. 4 showing a portion of the relaysimilar to the illustration in FIG. 2 to clearly illustrate the presentconstruction;

FIG. 6 is a top plan view similar to FIGS. 2 and 5 illustrating analternative construction particularly useful and adapted to alternatingcurrent responsive circuits; and

FIG. 7 is a view similar to FIG. 4 of an alternating current relay shownin FIG. 6 and construction in accordance with the present invention.

Referring to the drawings and particularly to FIG. 1, a welding circuitis schematically shown including an electrode 1 held in spaced relationto a workpiece 2. A power source 3, shown in block diagram as a suitableD.C. (direct current) source connected by leads 4 to the electrode 1 andthe work 2. A starting switch 5 is inserted in one of the leads 4 andwhen closed establishes a suitable voltage and power across theelectrode 1 and work 2 to establish and maintain an are 6 therebetween.A switch assembly '7 constructed in accordance with the presentinvention is shown mounted within the power source 3 and is connected incircuit to control operation of a timer or suitable recording device 8.As more fully described hereinafter, the switch assembly 7 normallymaintains the timer 8 deenergized. When switch 5 is closed to establishand maintain are 6, the current flow in the leads 4 actuates the switchassembly 7 to complete the circuit to the timer 8 and thereby make anaccurate record of the welding or load time.

Referring to FIGS. 13, the illustrated switch assembly 7 includes aninsulating mounting base 9 with a mounting arm or wall 10 projectingperpendicularly therefrom. In the illustrated embodiment of theinvention, the wall 18 is shown as a separate member interconnected tothe base 9 by suitable bolts 11. The base 9 and wall 10 may, if desired,be integrally formed. The base 9 is secured in any suitable mannerwithin the power source 3 with the wall 10 projecting horizontallytherefrom.

A generally U-shaped frame 12 formed of magnetic material has one armsecured to the wall 13 as by suitable screws 13 which pass through thewall and thread into suitably tapped openings in a side arm of theU-shaped frame 12. The frame 12 is mounted with the common web or baseof the member disposed in slightly spaced relation to the base 9 as at14 in FIG. 3. A bridging plate 15 of megnatic material spans the openend of the frame 12 with the one end of the plate 15 aligned with theinner arm 16 of frame 12 and the opposite end projected outwardly beyondthe outer arm 17 of frame 12. The plate 15 adjacent the outer arm isprovided with suitable edge notches 18 aligned and mating withupstanding or outwardly projecting ears 19 integrally formed on theopposite edges of the outer arm of member 12. A small U-shaped bracket20 is bolted as by bolts 21 or otherwise secured to the exterior of theouter arm 17 with an extension of the plate 15 disposed generallycentrally thereof. A coil spring 22 is disposed between the extension ofplate 15 and the inner surface of the outer arm of bracket 20 andcontinuously urges the plate 15 to pivot on the outer edge of the arm17. Suitable locating bosses 23 and 24 are shown respectively on theextension of plate 15 and the arm portion of the bracket 20 to align andhold the spring 22 in place.

An L-shaped stop arm 25 is secured to the outer wall 11) by the screws13 for frame 12 and projects inwardly over the upper edge of wall 10 andthe outer or free end of the plate 15. A stop screw 26 threads through asuitable opening in the arm 25 and serves as a stop for the plate 15 forlimiting the outward pivotal movement thereof under the action of coilspring 22. A lock or positioning nut 27 is threaded onto the stop screw26 and serves as an adjustable positioning or locating member for screw26. The frame 12 and plate 15 form a generally rectangular magnetizablemember or unit having the one movable side pivotally mounted or attachedto an adjacent side and with an air gap formed between the opposite andfree end of the movable side and the adjacent side of the unit.

A conducting strap 28 is formed of a strip of copper or the like and isconnected in series and forms a main current carrying portion of one ofthe leads 4. The strap 28 projects through the passageway defined byframe 12 and is secured to the base 9 by suitable attachment bolts 29and 30 which pass downwardly through suitable openings in the strap onopposite sides of the frame 11 and thread into suitably tapped openingsin the base 9. Suitable insulating spacers 31 encircle the respectivebolts 29 and 30 between the strap 28 and the base 9 to properly locatethe strap with respect to frame 12. As most clearly shown in FIG. 3, thestrap 28 is located adjacent to but slightly spaced from the web portionof the U- shaped magnetic frame 12. A centrally located disc 32 is shownfixed to the base of frame 12 in alignment with strap 28.

Generally, the coil spring 22 holds the plate 15 with the free endspaced from the arm 16 of the frame 12. However, when the arc 6 isstruck the current through the strap 28 establishes a magnetic fieldwhich is concentrated in the frame 12 and the plate 15. As a result, thefree end of plate 15 is pulled into engagement with the arm 16 andcompletes the magnetic path therethrough. The movement of arm 16actuates the switch assembly 7 and results in energization of timer 8,as follows.

L-shaped contacts 33 and 34 are secured to the opposite edges of thewall 10 with suitable bent over portions projecting slightly inwardly.Leads 35 and 36 are connected respectively to the contacts 33 and 34 andin series with the timer 8 to a suitable source of power for operatingtimer 8. A contact plate 37 is secured to the plate 15 as hereinafterdescribed. Contact plate 37 includes a bifurcated end defining contactcarrying arms 38 and 39 extending to either side of the wall 10 andterminating in alignment with the bent over portions of the contacts 33and 34. When the contact plate 37 engages contacts 33 and 34, thecurrent path therebetween is completed and operates the timer 8.

The contact plate 37 is secured to the plate 15 in the following mannerin the illustrated embodiment of the invention.

An insulating spacer 40 is mounted between plates 15 and 37 and includesa tubular boss 41 projecting through an opening in the central portionof plate 37. A stud bolt 42 passes through the boss 41 and a clampingnut 43 is threaded thereon to clamp the spacer 40 to the plate 15. Asmall coil spring 44 encircles the boss 41 and is compressed between thenut 43 and the contact plate 37 to resiliently hold the contact plate 37in position with the contact arms 38 and 39 in alignment with thecontacts 33 and 34.

A small lip 45 is provided integrally formed on the back edge of thecontact plate 37 and mates with a recess or opening 46 in the adjacentportion of the spacer 49 to guide and hold the contact plate inposition.

In summary, the illustrated embodiment of the invention operates asfollows. The start switch 5 is closed to establish and maintain the are6 between the electrode 7 1 and the work 2. The load current passesthrough the conducting strap 28 and establishes a corresponding magneticfield in the magnetic frame 12. The magnetic forces attract the plate 15and cause it to pivot, counterclockwise as viewed in FIG. 3, against theforce of the coil spring 22 to close the gap between the free end oftheplate 15 and the corresponding arm 16 of the frame 12. This pivotalmovement of the plate 15 similarly carries the contact plate 37 untilthe contact arms 38 and 39 engage contacts 33 and 34. The final movementof plate 15 further compresses the coil spring 44 and resiliently holdsthe arms 38 and 39 in engagement with the contacts 33 and 34. As aresult, a conductive path is established between contacts 33 and 34 andthe timer 8 is energized to record the welding operation.

When the are 6 is broken and the current through the strap 28 drops tozero, the magnetic field in frame 12 disappears and coil spring 22pivots the plate 15 in a clockwise direction as viewed in FIG. 3. As aresult, the contact plate 37 moves outwardly and breaks the circuitbetween the contacts 33 and 34 to again deeriergize the timer 8.

The switch assembly of the present invention may also distinguishbetween certain welding processes and only record those employing apredetermined minimum welding current. For example, the stop screw 26determines the spacing of the free end of the plate 15 from the adjacentarm 16 of the frame 12. As the spacing is increased a greater current isrequired to establish a magnetic field of suflicient strength to attractthe plate 15 sufficiently to bridge contacts 33 and 34. The tension ofthe coil spring 22, which opposes the pivoting of plate 15 by themagnetic field, can also be selected or set to restrict closure of theswitch unit until a minimum welding current passes through strap 28.

Although not shown, other electroresponsive devices may be employed withor in place of timer 8. An indicating lamp maybe connected to indicatewhen welding is being done. Or, a counter may be inserted in the circuitto record the separate welding operations made.

device for recording or otherwise detecting changes in arc welding orother similar current circuits, applicant has found that the relaystructure shown in FIGS. 4 and 5 provides an even more reliable andversatile relay unit for direct current circuits.

The relay of FIGS. 4 and 5 substantially corresponds to that illustratedin FIGS. 1-3 and the corresponding elements in the two embodiments aregenerally similarly numbered with the following description given toclearly illustrate and describe the modifications and improvements.

Referring particularly to FIG. 4, the embodiment of FIGS. 4 and 5includes a U-shaped core 12 of a crosssection generally similar to thatshown in FIG. 3. However, the core or frame 12 of FIGS. 4 and 5 is builtup of a plurality of individual U-shaped laminations 47 disposed instacked or nested relation and interconnected by a plurality of rivets48. In the illustrated embodiment of the invention, a pair of rivets 48is secured to the outer ends of each arm 16 and 17 of the U-shaped frame12 and four rivets 48 are distributed in a symmetrical relation in thebase portion. A bridging plate is shown including a pair of laminations49 interconnected by a plurality of rivets 50. The core plate 15 ispivotally mounted adjacent the arm 17 of frame 12 by an improvedmounting means including a mounting shaft or rod 51 which extendsthrough a passageway formed by a raised portion 52 in the outerlamination 49 of bridging plate or member 15. A hearing plate 53 issecured to the outer face of the arm 17 by a pair of screws 54 or thelike. The plate 53 includes a pair of upstanding ears or flanges 55which project outwardly to either side of member 15 and are providedwith properly located apertures to receive the mounting rod 51; Lockpins 56 shown as conventional cotter keys are passed through appropriateopenings in the ends of rod 51 to hold it in place. As in FIGS. 1through 3, a load conductor 57 is mounted extending through thegenerally rectangular core unit formed by the U-shaped frame 12 and thebridging plate or member 15.

In accordance with the embodiment of the invention illustrated in FIGS.4 and 5, the free end of the laminated member 15 is aligned with the endface of arm 16. However, in the embodiment of the invention shown inFIGS. 4 and 5, rivets 58 are secured to the member 15 and projectoutwardly from the innermost face thereof in alignment with the arm 16.The rivets 58 are formed of a suitable nonmagnetic material such asbrass. When the conductor 57 is energized and the member 15 is attractedto engage arm 16 of core frame 12, a selected air gap is maintainedtherebetween.

The contact plate 37 is mounted to the bridging member 15 for selectiveinterengagement of the contacts at the ends of contact arms 38 and 39with contacts 33 and 34, as in FIGS. 1 through 3, inclusive.

Generally, the operation of the embodiment of the invention illustratedin FIGS. 4 and 5 will correspond to that shown in FIGS. 1-3. Loadcurrent in the conductor 57 creates a magnetic flux within the generallyrectangular core formed by laminated members 12 and 15 which attract thebridging member 15. At a selected level, the magnetic force overcomesthat of the coil spring 22 and closes the air gap. As a result, thecontacts engage and provide a conductive path between contacts 33 and34. When the current drops to zero or some other selected level, themagnetic field disappears or decreases sutficiently to allow the coilspring 22 to pivot bridging member 15 and open the contacts. Thenonmagnetic rivets 58 or other similar spacing means which might be usedassure a positive and rapid opening of the circuit to insure reliableand long-life operation of the system. The laminated core 17 includingthe stacked lamination 47 provides a very simple and convenient meansfor adjusting the current range of the relay unit. Thus, by reducing thenumber of laminations in the core, it will provide direct relationshipwith the current ranges being operated upon.

The previous embodiment-s are shown applied to a direct current circuit.A preferred construction for an alternating current relay unit inaccodance with this invention is shown in FIGS. 6 and 7.

In FIGS. 6 and 7, a pair of alternating current relays 59 and 60,constructed in accordance with the present invention, is shown carriedby a suitable mounting board 61 or the like. The relays 59 and 60 aresimilarly constructed and the relay 59 will be specifically describedand corresponding elements of relays 60 identified by similar primednumbers. The relay structures of FIGS. 6 and 7 also substantiallycorrespond to that of the previous embodiments and the elementscorresponding to those of FIGS. 1-3 are similarly numbered and only themodifications are described.

Referring particularly to FIGS. 6 and 7, the AC. relay 59 includes theU-shaped frame 12 formed with a plurality of laminations 62 generally inaccordance with the illustration and teaching shown in FIGS. 4 and 5.Similarly, the bridging plate or member 15 is formed of a pair oflaminations 63 and is pivotally mounted adjacent the one end of theU-shaped core 12 by an adjustable pivotal mounting assembly 64corresponding to that shown in FIGS. 4 and 5.

A contact unit 65 is also similarly mounted on the opposite or free endof the bridging member 15, as shown in FIG. 1.

In the illustrated embodiment of the invention shown in FIGS. 6 and 7,however, the switch mounting wall 10 to which the one arm 16 of the coreframe 12 is secured as in FIGS. 15 is extended below the base of frame12 to provide a suitable enlarged Winding space 66 between the base ofthe frame 12 and the mounting board 61. One or more insulation wraps 67are wound upon the base and an alternating current winding 68 is woundon the base. As most clearly shown in FIG. 6, relay 59 is provided witha single turn winding 68 Whereas the corresponding winding 68' of relay60 includes two turns.

If the relay is employed with substantial line currents, a straightthrough winding such as shown in the direct current relay of FIGS. 1-3may be used. The number of turns provides a convenient and simple meansof controlling the current range for which the device is operable.Further, the number of turns is minimal in most practical ranges andconsequently the power consumed by the relay is inconsequential.

In FIGS. 6 and 7, a small slot or groove 69 is formed in the outermostend of the arm 16 and the mounting wall 10 is recessed immediatelyadjacent the outer end of the arm 16, as at 70. A conducting brass strapor ring 71 has one end disposed within the groove 69 and encirclesone-half of the arm 16. The ring 71 forms a short circuited single turnwhich provides highly reliable and silent operation of the alternatingcurrent relay. Further, as most clearly shown in FIG. 6, the spacingrivets of FIGS. 4 and 5 have been eliminated. However, member 15 mayinclude the rivet holes to provide versatility of assembling such thatthe same member 15 may be employed for both the direct current and thealternating current relays. Similarly, the modification to the mountingplate and the outer end of the arm 16 can be made without any change inthe basic structure of the assembly.

Generally, the alternating current relay operates in substantially thesame manner as the direct current relay with the relay pulling inaccordance with the adjustment or setting of the spring 22 and the stopscrew 26. Additionally, the precise current level at which the relaysare actuated will be controlled by the number of turns in the windings.In operation, it has been found that the relays of FIGS. 6 and 7 areclosed and opened with a rapid, snap action. Further, once the relay hasbeen energized or closed, it is held closed without any vibration of theplate member 15 as a result of the short circuited conductor 71 on thecore arm 16 of member 12.

The present invention provides a very rugged and reliable structureparticularly adapted for shop practice such as welding and similarprocessing shops.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. A load detecting apparatus adapted to respond to the current in aload circuit, comprising a generally rectangular magnetizable memberhaving a main laminated U-shaped frame and a movable side pivotallymounted at one end to an adjacent arm of the frame, said main laminatedU-shaped frame including a greater plurality of laminations than saidmovable side,

a spring guide secured to the arm of the frame and defining a wallparallel to the movable side,

a resilient means disposed between the movable side and the wall andurging said movable side to pivot outwardly and establish an air gapbetween the free end thereof and the aligned arm of the member,

adjustable stop means adjacent the free end to preset the length of theair gap between the movable side,

a conductor passing through said magnetizable member and adapted to beconnected in series in the load circuit, current in said conductorestablishing a magnetic field in said magnetizable mem-ber attractingsaid movable side,

a pair of spaced contacts mounted adjacent the free end of the movableside,

a contact plate having contact arms aligned with the spaced contacts andsuperimposed over the movable side of the magnetizable member,

an insulating spacer disposed between the contact plate and the movableside and having a tubular projection extending through an opening in thecontact plate,

bolt means passing through the tubular projection and clamping saidspacer to the movable side of the magnetizable member,

a coil spring encircling said projection and compressed between thecontact plate and the outer end of the bolt means, and

means to prevent relative rotational movement of the spacer and thecontact plate on said bolt means.

2. The relay apparatus of claim 1 having nonmagnetic rivets securedwithin openings in the free end of the movable side in alignment withthe adjacent arm of the frame and extending from the inner face toestablish a selected minimum spacing between the free end of the movableside and the adjacent arm.

References Cited by the Examiner UNITED STATES PATENTS 1,108,320 8/1914Berdon 307- X 1,503,980 8/1924 Carichoff 317-198 1,852,614 4/ 1932Johnson et al.

1,934,264 11/1933 Hefner 340-253 2,157,844 5/1939 Armstrong et a1.317-184 2,159,837 5/1939 Bab-b et al 317-184 X 2,341,896 2/ 1944 Bellowset al. 200-87 2,517,052 8/1950 Swinehart 200-87 2,528,744 11/1950 Fehr340-253 2,584,901 2/1952 Miller et al. 317-184 X 2,619,569 11/1952Savage 200-87 2,749,485 6/1956 Ober et a1 317-198 X 2,918,547 12/1959Titus 317-198 X 3,177,480 4/1965 Sankey 340-253 FOREIGN PATENTS1,164,591 10/1958 France.

NEIL C. READ, Primary Examiner.

D. K. MYER, Assistant Examiner.

1. A LOAD DETECTING APPARATUS ADAPTED TO RESPOND TO THE CURRENT IN ALOAD CIRCUIT, COMPRISING A GENERALLY RECTANGULAR MAGNETIZABLE MEMBERHAVING A MAIN LAMINATED U-SHAPED FRAME AND A MOVABLE SIDE PIVOTALLYMOUNTED AT ONE END TO AN ADJACENT ARM OF THE FRAME, SAID MAIN LAMINATEDU-SHAPED FRAME INCLUDING A GREATER PLURALITY OF LAMINATIONS THAN SAIDMOVABLE SIDE, A SPRING GUIDE SECURED TO THE ARM OF THE FRAME ANDDEFINING A WALL PARALLEL TO THE MOVABLE SIDE, A RESILIENT MEANS DISPOSEDBETWEEN THE MOVABLE SIDE AND THE WALL AND URGING SAID MOVABLE SIDE TOPIVOT OUTWARDLY AND ESTABLISH AN AIR GAP BETWEEN THE FREE END THEREOFAND THE ALIGNED ARM OF THE MEMBER, ADJUSTABLE STOP MEANS ADJACENT THEFREE END TO PRESET THE LENGTH OF THE AIR GAP BETWEEN THE MOVABLE SIDE, ACONDUCTOR PASSING THROUGH SAID MAGNETIZABLE MEMBER AND ADAPTED TO BECONNECTED IN SERIES IN THE LOAD CIRCUIT, CURRENT IN SAID CONDUCTORESTABLISHING A MAGNETIC FIELD IN SAID MAGNETIZABLE MEMBER ATTRACTINGSAID MOVABLE SIDE, A PAIR OF SPACED CONTACTS MOUNTED ADJACENT THE FREEEND OF THE MOVABLE SIDE, A CONTACT PLATE HAVING CONTACT ARMS ALIGNEDWITH THE SPACED CONTACTS AND SUPERIMPOSED OVER THE MOVABLE SIDE OF THEMAGNETIZABLE MEMBER, AN INSULATING SPACER DISPOSED BETWEEN THE CONTACTPLATE AND THE MOVABLE SIDE AND HAVING A TUBULAR PROJECTION EXTENDINGTHROUGH AN OPENING IN THE CONTACT PLATE, BOLT MEANS PASSING THROUGH THETUBULAR PROJECTION AND CLAMPING SAID SPACER TO THE MOVABLE SIDE OF THEMAGNETIZABLE MEMBER, A COIL SPRING ENCIRCLING SAID PROJECTION ANDCOMPRESSED BETWEEN THE CONTACT PLATE AND THE OUTER END OF THE BOLTMEANS, AND MEANS TO PREVENT RELATIVE ROTATIONAL MOVEMENT OF THE SPACERAND THE CONTACT PLATE ON SAID BOLT MEANS.